Because of their excellent characteristics, such as high hardness, high thermal conductivity, high light transmittance, and wide band gap, diamonds are widely used as materials of various tools, optical components, semiconductors, and electronic components and will become more important in the future.
In addition to natural diamonds, synthetic diamonds having stable quality are mainly used in industrial applications. Presently, most synthetic diamond single crystals are industrially synthesized under conditions where diamonds exist in a stable state, that is, at a temperature in the range of approximately one-thousand one-hundred to two-thousand nine-hundred degrees centigrade and at a pressure of at least tens of thousands of atmospheres. In addition to the high-temperature high-pressure synthesis method, another established diamond synthesis method is a gas-phase synthesis method. With recent advances in production techniques for diamond single crystals produced using a gas-phase synthesis method, that is, chemical vapor deposition (CVD) diamond single crystals, diamonds for use in tool and optical applications, as well as jewelry have been commercially produced, as described in Non Patent Literature 1 and Patent Literature 1.
For example, single crystal diamond tools are used in mirror finish and precision processing of nonferrous metals, such as aluminum alloys and copper alloys, and plastics, such as acrylics, for use in automobiles, optical apparatuses, and electronic equipment. Since single crystal diamond tools are generally more expensive than sintered diamond tools, which are often used in relatively rough processing, inexpensive and improved single crystal diamond tools have been proposed in Patent Literature 2, for example.
In industrial applications of diamond single crystals, relatively large millimeter- or centimeter-size single crystals are mostly used in cutting tools, such as bits, dressers, and end mills. These diamond single crystals are used in industrial basic processes, such as dressing of whetstones, ultraprecision machining of nonferrous metals, and mirror finish of resins. Thus, it is important to constantly and stably supply diamond single crystals.
Single crystal diamond tools, such as cutting tools (bits), dressers, and end mills, are used in industrial basic processes, as described above. Thus, it is important to stably supply diamond single crystals for use in these single crystal diamond tools.
Fluctuation in the price and amount of commercial natural diamond single crystals, which account for most of the industrial diamonds, is a factor contrary to stable supply. Because of the limited reserve of natural diamonds, the reserve of natural diamonds is decreasing with mining. A decrease in the reserve of natural diamonds may cause an increase in price or a decrease in the amount of commercial natural diamond single crystals. Thus, the role of synthetic diamond single crystals is expected to become more important.
Furthermore, natural diamond single crystals will be gradually substituted by CVD diamond single crystals. However, in industrial applications, particularly in cutting tool applications, existing CVD diamond single crystals are not as widespread as high-pressure synthesized diamond single crystals. This is because CVD diamond single crystals have lower toughness than natural diamond single crystals and high-temperature and high-pressure synthesized Ib-type diamond single crystals, and there is the technical problem that CVD diamond single crystals are difficult to process into the shape of a tool, such as a bit or an end mill, or chips or cracks tend to form during processing of a workpiece. This is similar to the situation where high-temperature and high-pressure synthesized IIa-type diamond single crystals, which are relatively brittle because of a small amount of nitrogen impurity in the crystals as compared with high-temperature and high-pressure synthesized Ib-type diamond single crystals, are rarely used in cutting tool applications in spite of their possibilities of relatively high hardness and long life.